Project Summary: Despite decades of research into the mechanisms of cardiac arrhythmias, both lethal and non-lethal manifestations of disrupted cardiac disruption continue to take the lives of hundreds of thousands of Americans and impose staggering financial costs [1]. The proposed project seeks to resolve a controversy in the field of cardiac electrophysiology regarding the roles extracellular spaces play in cardiac conduction. Briefly, gap junctional coupling is regarded as the primary mechanism of cardiac conduction, by which electrical signals in the heart propagate from cell to cell in a manner similar to electrical cables[2-4]. Recently, our lab has investigated a potentially complementary mechanism known as ephaptic coupling [5-8] which, when disrupted, can unmask diseases related to gap junctional uncoupling [9, 10]. Ephaptic coupling has been demonstrated to be modulated by the structure of the gap-junction- adjacent perinexus nanodomain and the ionic composition of the fluid in that space [9-13]. This evidence suggests that these primary factors of ephaptic coupling can be modulated by a method as simple and inexpensive as changing the ions contained in a bag of IV fluid to which a patient is connected in a hospital or ambulance. However, there is still debate in the field as to which mechanisms are affected, and in what ways, by extracellular expansion or collapse, both of which occur in a myriad of disease states [14, 15]. Interstitial expansion has been shown to both increase [3] and decrease [13] cardiac conduction velocity, by mechanisms attributed to cable and ephaptic theories, respectively. The proposed study aims to identify key mechanisms affected by interventions demonstrated in both of the above studies, specifically the expansion of bulk interstitium which would improve cable-like conduction and the narrowing of the perinexus, which would theoretically enhance ephaptic coupling. Identifying which parameters ? interstitial volume and perinexal width ? correlate with improved conduction will provide the field with targetable mechanisms of conduction preservation in the case of edema or ischemic events.